Studies of the mode of action of xylamine (XYL), an irreversible inhibitor of catecholamine uptake, are proposed. Xylamine (N-2'-chloroethyl-N-ethyl-2-methylbenzylamine) has been synthesized in these laboratories and shows promise as a probe for the study of neuronal uptake of norepinephrine (NE). The studies will utilize rabbit aorta, rat brain and superior cervical ganglia in in vitro experiments where, after prior exposure to XYL, 3H-NE accumulation and metabolism will be examined. In addition, experiments with 3H-labelled xylamine will be conducted to assess specific binding and its relationship to pharmacological effect. Recent in vivo studies with rats have shown that XYL causes a long lasting depletion of NE from the cortex and hypothalamus. The uptake of NE is also reduced in these tissues. Similar observations were made in the heart but the duration of depletion and uptake inhibition was shorter (at least 45 days for the brain and less than 1 week for the heart). In vitro experiments were consistent with these observations in that NE uptake could be shown to be inhibited after XYL was removed from the incubation medium. The irreversible actions of XYL could be protected against by coincubation with amphetamine, cocaine and NE, agents that act reversibly with the NE carrier. Chemical studies show that the inhibitory actions on NE uptake have been shown to be due to the aziridinium ion formed by XYL in solution. The effect on the CNS indicates that XYL penetrates the blood brain barrier presumably as the tertiary nitrogen mustard.